Technical, Economic, and Greenhouse Gas Reduction Potential of Combined Ethanol Fermentation and Biofuel Gasification-Synthesis at Sulphite Pulping Mills

The integrated production of second generation biofuel at a sulphite mill, by ethanol fermentation of spent sulphite liquor (SSL), can be expanded by gasification-synthesis to produce synthetic fuels from bark residues available onsite. The potential contributions of such synthetic fuel production to economic and environmental performance of the integrated facility, in comparison to ethanol production that is accompanied by heat and power generation from bark, were determined. Thus, bark is used either for (i) direct heat and power generation via direct combustion in a high pressure boiler or a biomass integrated gasification and combined cycle (BIGCC) or, alternatively, for (ii) gasification-synthesis to produce either methanol or Fischer-Tropsch (FT) synthesis crude (syncrude). Techno-economic evaluations and calculations of the net greenhouse gas reduction (NGGRP) potential for all process scenarios were conducted on the basis of Aspen Plus<sup>⊙</sup> simulations. Combining synthetic fuels production with SSL-fermentation at sulphite pulp mills were shown to improve the liquid fuel yields from 15 to 69 MW<sub>th</sub> on average, thus providing both acceptable NGGRP and investment opportunities that are financially viable. The internal rates of return (IRRs) were shown to improve from 25% for ethanol production only to 30% when combined with FT crude synthesis and to 32% when combined with methanol synthesis. Thus, negating disposal costs by utilizing waste biomass for combining synthesis fuels with ethanol production from SSL could be profitable at small scales, while maintaining the desired environmental benefits.